I have had the privilege of seeing F-14s and F/A-18s do flybys at sea, and let me tell you, at mach 1 there is NO WAY you will mistake that it was supersonic. As far as how low this guy was, it looks a little bit on the dangerous side to me, especially for the folks in the boats. I suspect it was part of an approved flight plan for an air show, although where it might have been is hard to say. Not in the US, at least I doubt it very much.